Formaldehyde and other contaminants, mainly acetaldehyde, are generated in considerable amounts during resin treatment steps such as blow-extrusion and blow-injection.
The residual formaldehyde content of the products can exceed 1000 ppb and the formaldehyde generation rate, measured at 290.degree. C., can be higher than 500 ppb/minute.
The presence of formaldehyde in drinks and/or food, like the presence of other contaminants, negatively affects their organoleptic properties, specifically their taste.
Accordingly, it is desirable to have the lowest possible formaldehyde content in the resin that makes contact with food or beverages.
It is also desirable for articles which do not make contact with food or beverages also to have a low residual formaldehyde content.
In order to obtain articles having a low residual formaldehyde content, the initial resin must be characterized not only by a low aldehyde content but also by a low rate of aldehyde generation at the process temperatures used to prepare the articles.
Methods are known for obtaining polyester resins with a low acetaldehyde content and with a low acetaldehyde regeneration rate (EP-B-41035).
The polyester resins meant for blow-extrusion, blow-injection or other processes have intrinsic viscosities usually above 0.75 dl/g. These values are obtained by subjecting the resin, after its polycondensation reaction (in which the intrinsic viscosity generally does not reach values higher than 0.6-0.7 dl/g) to solid-state polycondensation (SSP).
The SSP reaction is performed by heating the pellets of the resin, after its crystallization, to a temperature from about 180.degree. to 230.degree. C. for a time sufficient to obtain the intended IV value.
The process is performed in a stream of inert gas (nitrogen or other gases).
Extrusion of the resin to obtain the pellets to be subjected to SSP is generally performed in air. Resin processing to obtain the various articles is also performed in air.